Automating External Freeze on server with restricted access

Good Morning and thanks in advance for all replies.  I have created a PowerShell script for backing up CACHE and all related files to be run on a nightly basis.  My script works as long as the Unknownuser has %all access which is not acceptable in our production environment.  I have tried every format I can thank of to get the script to use a specific id with no luck.  How can I get this command to run under a specific id? 

..\bin\cache -s. -U%SYS "##Class(Backup.General).ExternalFreeze() <d:\backup\login.scr" 

The file just has

userid`r`n
​password`r`n

`r. For a carriage return and a new line, use `r`n.